The invention relates to a brake unit for controlling the brake pressures in the wheel brakes of a road vehicle, the brake system of which is designed as a dual-circuit brake system with a hydraulic brake booster and which is equipped with an antilock system.
A brake unit of this type is known from German Offenlegungsschrift No. 3,015,729.
In this brake unit, two jointly actuable master cylinders, each assigned to one of the brake circuits of the vehicle, are arranged next to one another in a twin design, in order to achieve a shorter overall length, as compared with a tandem master cylinder. The master cylinders are designed as step pistons, the smaller piston steps of which limit the outlet-pressure spaces and the larger piston steps of which limit drive pressure spaces, into which can be fed an outlet pressure, proportional to the pedal travel or to the pedal force, of a brake valve, by which this pressure utilized for brake boosting is derived from an auxiliary-pressure source. The outlet-pressure spaces of the master cylinders and their drive pressure spaces are connected to one another by central channels passing through the master cylinder pistons in the axial direction. Thus, their piston steps of smaller diameter, limiting the outlet-pressure spaces, and their piston steps of larger diameter, limiting the drive pressure spaces, are respectively subjected to the outlet pressure of the brake valve. Brake boosting takes place as a result of the effect of this outlet pressure on the excess piston surfaces of the larger piston steps. Thus, when the auxiliary-pressure source is intact, the two brake circuits operate as a dynamic brake circuit.
In the event of failure of the auxiliary-pressure source, when the brake system is actuated, tappets, on which engages a yoke coupled to the brake pedal, are forced against valve bodies, and these are forced against valve seats, located on the same side as the outlet-pressure spaces, of the master cylinder pistons. The connection between the drive pressure spaces and the outlet-pressure spaces of the master cylinders is broken and the actuating force is transmitted directly, via the tappets, to the master cylinder pistons which are now displaceable by pedal force only. Thus, in the event of failure of the auxiliary-pressure source, the two brake circuits operate as static brake circuits.
For the antilock control, there are solenoid valves, in particular brake-pressure regulating valves designed as 3/3-way solenoid valves. These valves can be commanded to move from a basic position, namely the pressure built-up position, in which the outlet pressure of a master cylinder is fed into the respective connected wheel brake or wheel brakes, through a blocking position into a pressure reducing position, in which the wheel brake or wheel brakes connected to the particular master cylinder are connected to the brake fluid storage tank of the brake system and can be relieved towards this. This type of antilock control, using the so-called bleeding principle, is necessary because under normal circumstances, that is to say when the auxiliary-pressure source is intact, the brake circuits are operated dynamically.
If the brake-pressure regulating valves and the brake unit are combined together into a hydraulic control unit on the housing block, this being the most expedient both in production terms and to keep the ducting short, this constructional unit acquires a relatively large constructional volume, and this presents considerable problems when it comes to accommodating it in the confined engine space of the vehicle. Another disadvantage is that an antilock control working on the bleeding principle exhibits a relatively sluggish response behavior at low brake pressures. That is to say when the control starts at very low coefficients of adhesion between the roadway and the vehicle wheels subjected to the control, since the pressure drop critical for the pressure reduction rate, between a wheel brake subjected to the control and the brake fluid storage tank is correspondingly low.
The object of the invention is, therefore, to improve a brake unit of the type mentioned in the introduction for a vehicle with an antilock system, to the effect that a hydraulic control unit comprising the brake unit and devices for antilock control can be produced with advantageously small spatial dimensions, and increased sensitivity of the brake-pressure control resulting in a faster response behavior can be obtained.
According to the invention, this object is achieved by the features to be mentioned.
According to these, the master cylinders provided for brake pressure control in the brake circuits I and II are designed as static cylinders actuated by a drive cylinder which engages master cylinder pistons via a pivotable rocker. This results in a uniform transmission of the actuating forces to the master cylinder pistons being achieved according to a predetermined brake pressure distribution to the brake circuits. The hydraulic drive cylinder provided for driving these can be arranged directly, including the brake valve of the brake booster, between the master cylinders of the brake unit, without contributing to an increase in the overall length of the brake unit. The use of static master cylinders which allow brake pressure control on the principle of volume variation, namely enlargement for pressure reduction phases and diminution for pressure build-up phases in antilock control, in combination with the transmission of the actuating forces to the master cylinder pistons via a pivotable rocker, makes it possible to vary the brake pressure in one of the two brake circuits as a result of the displacement of the piston of its master cylinder, without any appreciable reaction on the other brake circuit occurring thereby. On the contrary, the other brake circuit remains actuated with the force selected via the brake valve and the drive cylinder. A counter cylinder provided for the displacement of the master cylinder piston of the brake circuit subjected to the control and the piston of which is coupled in terms of movement to the master cylinder piston can be incorporated in a simple way into the brake unit, for example in a lateral arrangement next to the particular master cylinder, without the overall length of the brake unit being increased. The activation of the regulating cylinder provided for the control by the high outlet pressure of the auxiliary-pressure source ensures that the control responds quickly and, in particular, even at low absolute values of the brake pressure, a pressure reduction phase of the antilock control can take place at a high pressure reduction rate.
The same also applies accordingly when the brake unit according to the invention is additionally designed for controlling pressure build-up and pressure reduction phases of a starting-slip control.
A regulating cylinder suitable both for the antilock control and for the slip/propulsion control is then advantageously designed as a double-acting drive cylinder.
The features also provide a constructionally simple design of the piston of a master cylinder of the brake unit which is assigned to a brake circuit which can be subjected both to antilock control and to slip/propulsion control.
The features also provide arrangements and simple designs of regulating cylinders for antilock control which ensure that the regulating forces are transmitted free of pull-out torque to the master cylinder pistons and which can be incorporated in the brake unit, with only a small amount of space required.
The same applies accordingly to the basic construction and alternative design of a counter-cylinder suitable for the antilock control, with an annular piston arranged coaxially relative to the drive cylinder.
The features also provide a simple design and arrangement, suitable in combination with the above-mentioned regulating cylinders for an antilock control, of a regulating cylinder which is suitable for an additional slip/propulsion control, but which requires only a slight increase in the overall length of the brake unit.
Such an increase in the overall length can be avoided in a simple way as a result of the designs of regulating cylinders suitable both for the antilock control and for the slip/propulsion control.
Using a restoring spring and a specific minimum stroke of the master cylinder pistons or from a minimum pedal travel, makes an additional contribution to the restoring force counteracting the actuating force of the master cylinders. Thus, it is possible in a simple way to obtain a better approximation of the installed front-axle/rear axle division of brake pressure to its ideal pattern and, to that extent, also an improvement in the braking decelerations obtainable, without loss of driving stability.
The space enclosed by the additional restoring spring which is appropriately designed as a helical spring, can be utilized to accommodate an electrical displacement sensor for generating output signals which are characteristic of the position of the master cylinder pisto of a brake circuit which can be subjected to the control. These position output signals in combination with the output signals from wheel-speed sensors monitoring the movement behavior of the braked and/or driven vehicle wheels, can be used to obtain the most effective possible control of pressure build-up and pressure reduction phases of the antilock control or of the slip/propulsion control.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.